Incertae sedis (Latin for "of uncertain placement")[1] is a term used for a taxonomic group where its broader relationships are unknown or undefined.[2] Alternatively, such groups are frequently referred to as "enigmatic taxa".[3] In the system of open nomenclature, uncertainty at specific taxonomic levels is indicated by incertae familiae (of uncertain family), incerti subordinis (of uncertain suborder), incerti ordinis (of uncertain order) and similar terms.[4]

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The fossil plant Paradinandra suecica could not be assigned to any family, but was placed incertae sedis within the order Ericales when described in 2001.[5]

The fossil Gluteus minimus, described in 1975, could not be assigned to any known animal phylum.[6] The genus is therefore incertae sedis within the kingdom Animalia.

While it was unclear to which order the New World vultures (family Cathartidae) should be assigned, they were placed in Aves incertae sedis.[7] It was later agreed to place them in a separate order, Cathartiformes.[8]

When formally naming a taxon, uncertainty about its taxonomic classification can be problematic. The International Code of Nomenclature for algae, fungi, and plants, stipulates that "species and subdivisions of genera must be assigned to genera, and infraspecific taxa must be assigned to species, because their names are combinations", but ranks higher than the genus may be assigned incertae sedis.[9]

...the removal of many genera from new and existing families into a state of incertae sedis. Their reduced status was attributed largely to poor or inadequate descriptions but it was accepted that some of the vagueness in the analysis was due to insufficient character states. It is also evident that a proportion of the characters used in the analysis, or their given states for particular taxa, were inappropriate or invalid. Additional complexity, and factors that have misled earlier authorities, are intrusion by extensive homoplasies, apparent character state reversals and convergent evolution.

If a formal phylogenetic analysis is conducted that does not include a certain taxon, the authors might choose to label the taxon incertae sedis instead of guessing its placement. This is particularly common when molecular phylogenies are generated, since tissue for many rare organisms is hard to obtain. It is also a common scenario when fossil taxa are included, since many fossils are defined based on partial information. For example, if the phylogeny was constructed using soft tissue and vertebrae as principal characters and the taxon in question is only known from a single tooth, it would be necessary to label it incertae sedis.[4]

In botany, a name is not validly published if it is not accepted by the author in the same publication.[9]Article 36.1 In zoology, a name proposed conditionally may be available under certain conditions.[1]Articles 11 and 15 For uncertainties at lower levels, the system of open nomenclature suggests that question marks be used to denote a questionable assignment.[4] For example, if a new species was given the specific epithetalbum by Anton and attributed with uncertainty to Agenus, it could be denoted "Agenus? album Anton (?Anton)"; the "(?Anton)" indicates the author that assigned the question mark.[4] So if Anton described Agenus album, and Bruno called the assignment into doubt, this could be denoted "Agenus? album (Anton) (?Bruno)", with the parentheses around Anton because the original assignment (to Agenus) was modified (to Agenus?) by Bruno.[4]